1. Field Of The Invention
The present invention relates generally to high-frequency band-pass filter apparatus and methods of fabricating such filters and, more particularly, to non-TEM-mode interdigital band-pass filters and methods of fabricating such filters.
2. Prior Art
In high-frequency communication systems, the band-pass filter is a necessary, and often essential, element. A well-known example of the band-pass filter is the wave guide filter. As is well known, signal propagation through the wave guide band-pass filter is in the non-TEM mode. The physical dimensions of the filter can be realized according to known techniques. The verb "realize" as used herein means the ability to calculate the physical dimensions of the filter from a desired electrical response so that the actual electrical response produced by the filter incorporating the calculated physical dimensions closely approximates the desired electrical response.
The wave guide band-pass filter, while achieving the desired electrical response, has several major practical disadvantages. Because the wave guide band-pass filter must always be a three-dimensional structure, it is physically difficult to construct and adjust for optimum performance and is very expensive to manufacture. Moreover, the wave guide band-pass filter using an air dielectric is very large, even at super high frequencies (SHF). For example, a wave guide band-pass filter with a center frequency f.sub.o of 1 gigahertz (GHz) typically occupies a volume of at least 6".times.6".times.36".
Another well-known form of the band-pass filter is the transverse electromagnetic (TEM) mode filter. One group of TEM-mode band-pass filters uses interdigital resonators disposed between first and second ground planes, and are referred to herein as TEM-mode interdigital band-pass filters. Examples of TEM-mode interdigital band-pass filters are shown in U.S. Pat. Nos. 3,327,255, Bolljahn, et al.; 3,348,173, Matthaei, et al.; and 4,020,428, Friend, et al. A basic reference is D. D. Grieg and H. F. Englemann, "Microstrip--A New Transmission Technique for the Kilomegacycle Range," Proc. I.R.E., Volume 40, December 1952, pages 1644-1650.
FIGS. 1 and 2 show two views of the TEM-mode interdigital band-pass filter. FIG. 1 is a top plan view of the filter with the top ground plane removed, whereas FIG. 2 is a side perspective view of the filter. As is taught by the Bolljahn, et al. patent, a TEM-mode interdigital band-pass filter having n filter sections (where n is a positive integer) requires n+2 interdigital resonators. As shown in FIGS. 1 and 2, each interdigital resonator a.sub.i (where i goes from 1 to n+2) is rectangular in shape and has a length of dimension e and a width of dimension c.sub.i. Each resonator a.sub.i has a very small depth and is typically made of electrically conductive foil. This form for resonators a.sub.i is called stripline, sandwich-line, and sometimes microstrip. However, it should be noted that the depth of each resonator a.sub.i can be increased so that each resonator a.sub.i is in the form of a cylinder, bar, etc.
Resonators a.sub.i are arranged in parallel so that they define a plane. The space separating each pair of adjacent resonators a.sub.i, a.sub.i+1 is of dimension d.sub.i,i+1. The left-most resonator a.sub.1 is the input resonator, and the right-most resonator a.sub.n+2 is the output resonator. Because the TEM-mode interdigital band-pass filter is electrically reciprocal, the input could be resonator a.sub.n+2 and the output could be resonator a.sub.1. Each resonator a.sub.i has one grounded end which is opposite to the grounded end of the adjacent resonators a.sub.i-1, a.sub.i+1. This sequence of electrical connection accounts for the use of the term interdigital in the art to describe this group of filters.
Each resonator a.sub.i has an electrical length approximately equal to one quarter wavelength (hereinafter, .lambda./4) of the center frequency f.sub.o of the passband of the filter. A first electrical ground plane 13 is provided a distance b above the plane defined by the resonators a.sub.i, and a second electrical ground plane 15 is provided a distance b below the defined plane. A dielectric 12 is provided between ground planes 13, 15. Typically, electrical side planes (not shown) are provided on either side of ground planes 13, 15 to provide the interdigital electrical connection as well as support to resonators a.sub.i.
Because the resonators a.sub.i are disposed the same distance b from ground plane 13 and from ground plane 15, the E field is completely symmetrical. This field symmetry has important ramifications. Coupling in the filter is produced by the fringing electromagnetic fields between the resonators a.sub.i. When a homogeneous dielectric 12 is used, the symmetrical field allows a filter to be designed in which only TEM-mode propagation occurs.
TEM-mode propagation allows the calculation of the dimensions b, c.sub.i, d.sub.i,i+1 and e of the TEM-mode interdigital band-pass filter using only several simple equations having closed-form solutions. One well-known design procedure is presented in Matthaei, Young and Jones, Microwave Filters, Impedance--Matching Newtorks, and Coupling Structures, McGraw-Hill Book Company, New York, 1964, at 10.06 and 10.07. A well-known improvement on the Matthaei, et al. procedure is found in E. G. Cristal, "New Design Equations for a Class of Microwave Filters," IEEE Transactions on Microwave Theory and Techniques, Volume MTT-19, No. 5, May 1971, pages 486-490.
The first step in the design of a TEM-mode interdigital band-pass filter using either the Matthaei, et al. or Cristal procedures is the selection of the desired electrical passband response parameters of: f.sub.o, the center frequency of the passband; .DELTA.f, the frequency size of the passband; .delta., the maximum ripple in dB in the passband; .OMEGA..sub.IN, the input impedance of the filter; and .OMEGA..sub.OUT, the output impedance of the filter. Next, the .delta. value is compared with charts referred to in both of the references to determine the minimum number of sections n (where n is a positive integer) that the filter must have. As is well known, the filter can have more than n sections to produce a lower .delta. value while still achieving a required amount of rejection at some frequency outside of the passband. The n value is then compared with additional charts referred to in both of the references to obtain the low-pass prototype values for the filter elements. These prototype values are normalized values. The desired .OMEGA..sub.IN and .OMEGA..sub.OUT are produced by appropriately multiplying the prototype values, as is well known in the art. The multiplied prototype values are then used to compute the self capacitance of each resonator a.sub.i and the mutual capacitance of each adjacent pair of resonators a.sub.i, a.sub.i+1 using either the Matthaei, et al. or Cristal procedures. The self and mutual capacitances are then used to calculate the b, c.sub.i, d.sub.i,i+1 and e physical dimensions of the TEM-mode interdigital band-pass filter.
There is considerable confusion in the art over the terms used to describe variations in physical structure of TEM-mode interdigital band-pass filters. When interdigital resonators a.sub.i have a very thin depth so that they are of the form of thin strips, the filter has been called a triplate or stripline filter. However, when the depth of the interdigital resonators a.sub.i increases to form a rod or bar, the filter has been called a sandwich, rod, or bar interdigital filter.
As stated in the patent references given above, especially Friend, et al., and as is well known in the art, the TEM-mode interdigital band-pass filter suffers from several major deficiencies. Because the filter requires that the interdigital resonators a.sub.i be disposed an equal distance b from each of the two ground planes, a support apparatus (not shown) for the interdigital resonators a.sub.i must be provided. When air or a gas is used as the homogeneous dielectric 12, the support apparatus becomes physically complex. When a solid dielectric is used as the homogeneous dielectric 12, the ground planes must be in tight physical contact at all points with the solid dielectric 12, lest non-TEM-mode propagation occurs. This tight physical contact requires numerous fasteners. A solid dielectric 12 is preferred over a gas dielectric 12 because of the very substantial size reduction in the filter that is achieved. Another major deficiency is the very close physical tolerances that are required. These tight tolerances require many manufacturing steps so as to achieve the desired electrical response. The tight tolerances result in substantial manufacturing and final optimization costs.
In order to overcome many of the disadvantages found in TEM-mode interdigital bandpass filters, it has been suggested that an interdigital bandpass filter be constructed having only one of the ground planes of the TEM-mode interdigital band-pass filter. By eliminating one of the ground planes, it was thought that manufacturing and optimization costs could be reduced substantially. See, T. A. Milligan, "Dimensions of Microstrip Coupled Lines and Interdigital Structures," IEEE MTT-25, No. 5, May 1977, pages 405-410.
An example of such a filter is shown in FIGS. 3 and 4, where FIG. 3 is a top plan view and FIG. 4 is an end view. Resonators R.sub.1 to R.sub.8 of a 6-section filter are disposed above a single electrical ground plane 22 by a solid homogeneous dielectric 20. The resonators R.sub.1 to R.sub.8 are connected in interdigital fashion by two electrical lines 23, 23', provided along the top and bottom edges of dielectric 20, as shown in FIG. 3. Each electrical line 23, 23' is connected to ground plane 22 by a separate electrical side strip 32, as shown in FIG. 4. Resonator R.sub.1 is the input and resonator R.sub.8 is the output. Because the filter is electrically reciprocal, resonator R.sub.8 could be the input and resonator R.sub.1 could be the output. Resonators R.sub.1 to R.sub.8, ground plane 22, electrical lines 23, 23' and side strips 21 are microstrip in the filter shown in FIGS. 3 and 4.
The elimination of one of the ground planes, however, means that the E field is no longer symmetrical in the filter shown in FIGS. 3 and 4. Thus, propagation is no longer in the TEM mode. As stated above, wave guide band-pass filters also operate in the non-TEM mode. However, the physical dimensions of a wave guide band-pass filter can be readily calculated because the nature of the propagation is very well understood and the design equations have closed-form solutions. The calculated physical dimensions result in a wave guide band-pass filter whose electrical response closely approximates the electrical response used to calculate the physical dimensions.
The non-TEM-mode propagation in the non-TEM-mode interdigital band-pass filter shown in FIGS. 3 and 4 prevents the use of the simple and closed-form design procedures of Matthaei, et al. or Cristal discussed above. The inventor, as well as others in the art, have used the Matthaei, et al. or Cristal procedures to calculate the physical dimensions of the non-TEM-mode interdigital band-pass filter. The calculated physical dimensions, however, produce a filter whose electrical response grossly deviates from the electrical response used to calculate the physical dimensions. Moreover, other design approaches also have failed. See the Milligan reference above. For this reason, it can be said that prior to the present invention, it has been impossible to realize a non-TEM-mode interdigital band-pass filter. The inventor has discovered a method of fabrication for determining the physical dimensions of a non-TEM-mode interdigital band-pass filter having one ground plane, whose electrical response closely approximates the electrical passband response used to determine the physical dimensions.